1. Field of the Invention
This invention relates to compositions that exhibit the Johnsen-Rahbeck effect. This effect is defined as the increase in frictional force between two electrodes in contact with a semiconductor that arises when a potential difference is applied between the electrodes.
2. Description of the Related Art
A description of the early work relating to the Johnsen-Rahbeck effect may be found in "Some Theoretical and Practical Considerations of the Johnsen-Rahbeck Effect" Audrey D. Stuckes, Proceedings of the Institution of Electrical Engineers, Vol. 103 (b), 1956, pages 125-131. This article indicates that two Danish scientists, Johnsen and Rahbeck, observed that when about 400 volts was applied between plane and polished plates of lithographic stone and metal, a force of about 50 grams per square centimeter developed between these plates. This force disappeared upon removal of the voltage. By systematic investigation, these Danish scientists found that this force could be obtained with a number of minerals, including agate, limestone and marble, as well as some organic materials such as ivory, horn, bone and cellophane. It was believed that the presence of moisture was essential because only very small forces were generated when the specimens were dried. The magnitude of the force was found to vary with the electrical conductivity of the material, the degree of polish that could be given to its surface, and the direction of the current flow. When the semiconductor was negative with respect to the metal, maximum force was obtained. Further, the Stuckes article indicates that the Johnsen-Rahbeck effect could find applications in electrostatic clutches having two flat, polished surfaces, one of metal and the other of a semiconductor, held lightly in contact. Since the force of attraction is inversely proportional to the square of the distance separating the surfaces, the two clutch surfaces must be almost in physical contact before current is applied. Consequently, it is desirable that one of the polished surfaces must be able to align itself relative to the other. Upon testing, the Stuckes article indicates that the clutch breaks down after a period of time and that in almost every case this breakdown was due to wear of the metal surface. The presence of even extremely small particles produced by wear drastically alters the gap between the surfaces thereby progressively reducing the torque and ultimately rendering the clutch inoperative.
There are several clutch-type devices that use this Johnsen-Rahbeck effect. For instance, U.S. Pat. No. 3,343,636 is directed to electrostatic clutches using the Johnsen-Rahbeck effect. One component of the clutch comprises a semiconductive member consisting of a surface layer of semiconductive material, which provides the engagable surface of the component and which is in contact either with a metal body forming the main part of the component or with an electrically conducting layer interposed between the semiconductive layer and the main body of the component, if the latter is of an insulating material. The second component of the clutch may be formed wholly of metal, or may include a metal member forming the engagable surface. The clutch is illustrated at FIGS. 1 and 2 of the patent and is composed of a plurality of separate sections, each individually mounted on a surface of the main body of the component so as to be capable of pivotal movement relative to the main body. The engagement of each of the sections is individually alienable with the engagable surface of the second component of the clutch, when the two components are in the disengaged position.
U.S. Pat. No. 4,393,967 also discloses an electrostatic clutch operable by the Johnsen-Rahbeck effect, which may be used to operate a print hammer device. The engagable surface of the drum is a semiconductive surface prepared by sputtering or vapor depositing a layer of substantially pure silicon carbide onto a conductive substrate. The resistance of the silicon carbon layer depends both on the resistivity of the silicon carbide material and the thickness of the layer. It is generally desirable that the thickness of the layer be between 10 and 2,500 micrometers, although it is possible to prepare useful clutches with silicon carbide layers of greater or lesser thickness.
At least some research has been carried out to test the utility of certain polymers in Johnsen-Rahbeck clutches. For instance, U.S. Pat. No. 3,721,649 discloses a semiconductive phthalein-fluoran polymer that, when used with an appropriate binder, such as a powdered phenolic resin precursor, is adapted for use in a friction element of an electrostatic device of the Johnsen-Rahbeck type. The patent defines semiconductors as materials that have electrical resistivity characteristics somewhere between those of insulators and metals, i.e., a resistivity between about 10.sup.2 and 10.sup.10 ohms-centimeters. The phthalein-fluoran semiconductor polymers are characterized as being semiconductors of the organic polymer type rather than the molecular crystal type. Semiconductive organic polymers are not well characterized, partly due to difficulty in establishing their physical-chemical structures. The patent further discloses that after prolonged periods of use, slipping between the semiconductor and metal surface causes slight wearing away of the semiconductor material. This wearing away has, in the prior art, produced dust particles which adhere to the contacting surfaces thereby accelerating wear of the semiconductive surface and shortening the useful life of the clutch. The phthalein-fluoran material is said to provide a combination of a friction element which wears during operation without the adverse production of dust particles. Any dust particles that may be produced are said to be "non-adherent" to the contacting surfaces and are "ejected" by the movement of the contacting surfaces.
U.S. Pat. No. 3,871,944 is directed to an integral composite element that is useful in electrostatic clutches or brake devices. The device disclosed includes a layer of particulate semiconductive material and a layer of highly conductive powdered metal. The entire element is integrated with a hard thermoset bonding resin. The semiconductive layer should be between 0.3 to 1.2 millimeters thick, preferably between 0.7 and 0.8 millimeters thick, to provide the appropriate Johnsen-Rahbeck effect.
There yet exists a need in the art for a semiconductive composition that, in combination with a metal opposite electrode, exhibits a high Johnsen-Rahbeck response; that can be conveniently used in doing work using the Johnsen-Rahbeck effect; and that minimizes wear of the contacting surfaces so that the Johnsen-Rahbeck device has a commercially useful service life.